1. Field of the Invention
This invention is directed to a work support means and process, for supporting and positioning a large workpiece for machining of the workpiece to complex shapes, such as by the use of a numerically controlled horizontal boring mill.
2. State of the Art
In the art of machining marine propellers of large size, up to 30 or more feet in diameter and weighing 80 tons or more, it is necessary to machine each propeller to precise dimensions in geometrically complex forms.
One way of machining large propellers, used commonly in the United States, is to cast a propeller in the general outline of its final form, using a hand made wood pattern for the casting form, to clean the casting, and to do the final finishing of the propeller by hand, using hand-operated machines, such as grinders, chipping tools, and the like. This is an extremely time-consuming and expensive operation, and the machining of a large propeller can often take as much as 4,000 man-hours to accomplish.
There are machines commercially available for the purpose of machining propellers, but these have very substantial shortcomings. One such machine is a specialized propeller milling machine which has a cost of approximately one-and-one-half million dollars, and utilizes a revolving table and revolving and rotating spindles to do the machining. However, this expensive machine does not produce an accurate surface finish in the final workpiece, and thus requires considerable hand finishing, does not efficiently utilize the available horse power because of the angle of orientation of its spindle, and is incapable of machining many parts of the propeller blade, such as the hub, fillets and blade outline.
Another commercially available boring machine which has nine axes of movement, uses two rotatable spindles with movable joints to machine the individual blades of variable pitch propellers. However, these machines are not capable of machining the propellers of a single unit blade, and are therefore only capable of being utilized in connection with a relatively small proportion of the marine propellers which are manufactured. Also, the use in this machine of movable joints results in a substantial loss of machine rigidity, reduces the ability of the machine to transmit power efficiently, and consequently results in a very inefficient use of machine power, reducing machining rates and increasing manufacturing costs. This machine is also extremely expensive, costing on the order of two million dollars.
Both of the foregoing commercially available machines are also limited in that they are primarily adapted for use in machining propeller blades. Therefore, if either machine is not being utilized for machining propeller blades, it is not readily adaptable for other general machining purposes, such as might be found in a conventional machine shop. This limited utility therefore substantially increases the expense of operating these machines, by reducing their efficiency, because of increased non-productive down time.
In the machining of large propellers and similar objects by a horizontal boring mill, it is necessary rigidly to support the workpiece, in order to prevent workpiece chatter or movement under the substantial forces of the boring mill spindle, which could produce an imperfect work surface requiring substantial hand finishing. In the prior art processes, the workpiece is rigidly supported by a substantial number of mounting blocks, built up under each propeller blade and to which the propeller blade is fastened by the use of finger clamps. These finger clamps, to be effective, must overlie the surface being machined and therefore interfere with the uninterrupted machining of the entire surface. This necessitates the movement of the finger clamps, at least once and often more than once, during the machining of each propeller blade. This is obviously time-consuming and therefore inefficient.